The present invention relates generally to the creation and maintenance of a vacuum in a chamber which is open to the atmosphere. This invention relates more particularly to supersonic steam aerodynamic windows.
For certain purposes, it is desired to fire corpuscular beams into gases of high pressure, as for example in electron beam welding. In such instances, the beams are formed in a high vacuum space, and possess either kinetic energies too low for the penetration of a thin foil solid window, or current density so high as to melt any such window. With a supersonic flow aerodynamic window, the pressure difference between the high vacuum space and the area of high pressure is supported by centrifugal forces induced by the curvature of a supersonic gas jet. Since the flow is supersonic, turning of the jet occurs across waves, and window designs using both expansion and compression waves have been considered. The flow rate required by these windows is an important measure of their efficiency, and depends on the window design and operating conditions. A more detailed description of supersonic flow aerodynamic windows can be found in the paper, "Dynamic Pressure Stages for High-Pressure/High-Vacuum Systems," by B. W. Schumacher, Ontario Research Foundation, Physics Research Report 5806 (1958), incorporated herein by reference.
As has been practiced in the art, a supersonic flow aerodynamic window operates by expanding a high pressure gas to supersonic speeds in a two-dimensional converging/diverging nozzle. It is known in the art to direct air or some other dry gas through a nozzle at an angle across the path of high pressure air entering through an orifice into a vacuum chamber. The dry supersonic air passing across the opening sets up a shock wave which functions as a barrier to stop the inflow of high pressure air. A disadvantage of this arrangement is that large compression or pumping systems are required in order to keep the vacuum chamber at a low enough pressure suitable for the application intended. Additionally, only openings on the order of 10 mm in width are permitted if the vacuum pumps are to maintain the pressure ratio desired between the high pressure region and the low pressure region. Paper 72-710 of the AIAA 6th Fluid and Plasma Physics Conference, Boston, Mass., June 26-28, 1972, entitled, "Supersonic Flow Aerodynamic Windows for High-Power Lasers," by E. M. Parmentier and R. A. Greenberg, incorporated herein by reference, more thoroughly discusses the use of supersonic jets to support a pressure difference between two cavities open to each other through an orifice.
A steam ejector typically operates by accelerating superheated steam under pressure through a convergent-divergent nozzle to emerge with a velocity which may exceed 3,000-4,000 feet/second. Air or gas in the vicinity of the point where the steam leaves the nozzle becomes entrained with the steam, and this entrainment creates a vacuum as the air or gas is pulled along with the steam. The vapor jet loses some of its velocity in sharing momentum with the air or gas which has been entrained, and the gas-vapor mixture thereafter decelerates progressively as the mixture travels along the ejector tube, losing kinetic energy and gaining pressure energy until, at the ejector outlet, the desired degree of compression has been achieved. The ejector tube is contoured to provide the appropriate flow area at every stage of the deceleration and compression, deceleration at supersonic speeds requiring a converging, and at subsonic speeds a diverging, passage. Steam ejectors are more completely described in High Vacuum Pumping Equipment, by B. D. Powers, Chapter 4, "Steam Ejectors" (1966), which is incorporated herein by reference.
It is a unique feature of this invention to use a steam ejector in place of a standard nozzle supplying dry gas across the aperture between the high and low pressure areas. Additionally, it is unique to mix superheated steam with the air in the high pressure area prior to admitting it through the aperture into the low pressure area. The combination of using the steam ejector as well as mixing superheated steam with the incoming atmospheric air provides much more efficient pumping action to keep the low pressure chamber in a vacuum state, and also permits use of a much larger aperture.
It is, accordingly, a general object of the invention to provide an improved apparatus and method for maintaining a vacuum in a chamber which has an opening to the atmosphere wherein an aerodynamic window using steam creates a barrier to limit the entry of atmospheric air passing through the opening.
It is another object to provide an improved apparatus and method for maintaining a vacuum in a chamber which is open through an orifice to atmospheric air, wherein an aerodynamic window using superheated stream creates a barrier to the opening whereby the apparatus can be used with a linear accelerator as the first stage of a differential vacuum pumping system for accommodating the passage of an accelerated electron beam into the atmosphere.
It is a further object to provide an improved apparatus and method for maintaining a vacuum in a chamber which has an opening to the atmosphere, wherein an aerodynamic window is created by colliding a jet of superheated steam and a jet of air/superheated steam mixture within the chamber to form a barrier, limiting communication of the atmosphere with the chamber; and further including a cooling system to condense the steam and remove the condensate from the chamber, thereby enhancing the vacuum condition of the chamber.
Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows; and in part will become apparent to those skilled in the art upon examination of the following; or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.